This invention relates to internal cooling within a gas turbine engine, and more particularly, to an assembly for providing more efficient and uniform cooling in an interface or transition region between a combustor liner and a transition duct.
Traditional gas turbine combustors use diffusion (i.e., non-premixed) combustion in which fuel and air enter the combustion chamber separately. The process of mixing and burning produces flame temperatures exceeding 3900° F. Since conventional combustors and/or transition pieces (or ducts) having liners are generally capable of withstanding a maximum temperature on the order of only about 1500° F. for about ten thousand hours (10,000 hrs), steps to protect the combustor and/or transition piece must be taken. Typically, this has been done by a combination of impingement and film-cooling which involves introducing relatively cool compressor discharge air into a plenum formed by a flow sleeve surrounding the outside of the combustor liner. In this prior arrangement, the air from the plenum passes through apertures in the combustor liner and impinges on the exterior liner surface and then passes as a film over the outer or cold-side surface of the liner.
Because advanced combustors premix the maximum possible amount of air with the fuel for NOx reduction, however, little or no cooling air is available, thereby making film-cooling of the combustor liner and transition piece problematic. Nevertheless, combustor liners require active cooling to maintain material temperatures below limits. In dry low NOx (DLN) emission systems, this cooling can only be supplied as cold side convection. Such cooling must be performed within the requirements of thermal gradients and pressure loss. Thus, means such as thermal barrier coatings in conjunction with “backside” cooling have been considered to protect the combustor liner and transition piece from damage due to excessive heat. Backside cooling involves passing the compressor discharge air over the outer surface of the transition piece and combustor liner prior to premixing the air with the fuel.
With respect to the combustor liner, another current practice is to impingement cool the liner, or to provide turbulators on the exterior surface of the liner (see, for example, U.S. Pat. No. 7,010,921). Turbulation works by providing a blunt body in the flow which disrupts the flow creating shear layers and high turbulence to enhance heat transfer on the surface. Another practice is to provide an array of concavities on the exterior or outside surface of the liner (see, for example, U.S. Pat. No. 6,098,397). Dimple concavities function by providing organized vortices that enhance flow mixing and scrub the surface to improve heat transfer. The various known techniques enhance heat transfer but with varying effects on thermal gradients and pressure losses.
There remains a need for more efficient and more uniform cooling at the combustor liner/transition piece seal interface, and for minimizing leakage at the interface seal where cooling air is routed to the seal region from a higher-pressure location for the purpose of cooling the seal and adjourning components.